Solar Power Solutions for Mobile Computing.

Refining the Solar equipment. Li-Ion or Lead-Acid?

I’ve just had a nice conversation with my energy advisors, Select Solar, about some of the issues I’ve seen with the tests I’ve done so far. The main problem being that Li-Ion batteries will not trickle charge. They require a certain level of stable voltage/current before they start to charge and this wastes the possibility of charging a battery at lower sun power. In fact, on a hazy day or with light cloud cover, the sun power might not actually go above 70% and you have no chance to charge anything. This could be major problem and I’ve decided to try and address it.

A solution to this problem, as I mentioned before, is lead-acid batteries. These are the type you will find in your car and are a tried, tested, reliable and relatively cheap solution. The problem is that they are also extremely heavy – at least 400% of the weight for like-for-like power storage. They also operate at 12v which means voltage conversions (and more electronics and power-wastage) in order to operate the UMPC.

The decision I’ve made is to buy a 70W/hr 12v lead-acid battery (2200g) and to try and use this as a charge buffer. Here’s a little sketch of the planned set-up.

The idea would be that I connect devices to the lead-acid battery as the solar power increases during the day. For example, I might add a couple of AA batteries or my mobile phone in the morning. At mid morning I might replace it with the Li-On battery and during the peak hours I could re-attach the AA battery charger with 2 or 4 batteries depending on sun power.

Two problems I can see here:

How do you know the charge on the lead-acid battery?

Will the lead-acid battery be able to feed up to 1.5 Amps?

There’s only one way to find out. I’ve just put an order in for a 70W/hr lead acid battery and charge controller along with some fresh AA batteries, a powerbank tip adaptor for the Samsung Q1, a 12v charger for the mobile phone and something I’ve always wanted, a flexible USB LED-lamp!!!

Later today I will probably be ordering the solar panel. I won’t be using the P3 Panel as the only advantage with that was that it could drive 19V into the PowerBank. Now that I’m going for a 12V source solution I’m going to be looking at the 25W version of this Sunlinq panel which is the same price as the 15W version of the P3 panel. There’s a risk that its going to be too big for the bike at 1m long but even if I fold 25% of it away, its still going to give more power than the 15W panel and during mid-day pause and work sessions I hope to get the full 25W out of it.

However, I don’t like the idea of using inverters. You take DC then store it in a battery (loss #1) and then convert it to 110v (loss #2) and then through the adaptor back to DC(loss #3) and then through the power board on the PC to the various voltages (loss #4) The inverters are also heavy, dangerous and a failure point thats just not needed.

The best method is to stick with low voltage DC I think. Most efficient would be to use the solar panel straight into the UMPC but I think thats too risky unless you can get someone to confirm that its safe. 2nd best solution would be to charge the Li-Ion battery and then use that to power the UMPC (remove the battery from the UMPC when doing this)If sun power is low then the lead-acid battery gives you the best option. Can also give you power to charge other devices (phones, AA batts and UMPC) with a stable voltage. I hate the idea of that 2.2KG lead acid battery but I think its a neccesary evil.

A 12V UMPC would be nice. There are some around. Raon Digital Vega for example. But its too small for me.